Audio frequency amplifier



w. R. KOCH 2,179,263

AUDIO FREQUENCY AMPLIFIER Filed Nov 30, 1936 +5 iaazzk g [44 comma/vPOM/67E a mom SUPPAY 6864427? fiscal/5e 54 26 $1 30 27 49 f 47 35 at 42LL 4g i 19.6. 7 ,z

26 5; 46 r 52 Ac ///97EK 6( 40 a F/Zfl/Vf/Vffi 7 FIELD 3v 3 nnentorattorney Patented Nov. 7, 1939 {TY-UNITED STATES PATENT OFFICE AUDIOFREQUENCY AMPLIFIER Winfield R2 Koch, Merchantville, N. J., assignor toRadio Corporation of America, a corporation of Delaware 2 ApplicationNovember 30, 1936, Serial No. 113,473

4 claims. (or. 179-171) v The present invention relates to audiofrequency amplifiers, and more particularly to audiofrequency-amplifiers in connection with radio receiving apparatus andthe like having a common power supply connection with the receiver,

.power amplifier stage of a radio receiver or the like whereby theamplifier stage maybe operated from a single power supply device Withoutundue interference or flutter in the receiver circuit.

It has been found that the tendency of a radio receiver to flutter maybe minimized considerably if the grid filter capacitorin the audioamplifier output stage which creates an interfering surge in responsetostrong signals, is returned to the' plate supply positive lead ratherthan to ground or. chassis.

The grid filter, with the filter capacitor returned to ground, hasnormally a time constant much slower thanthe time constant of the powersupply means, consequently the grid bias voltage tendsto remain constantduring a surge. If the grid filter capacitor is connected to a point atwhich the surge originates, a counter-voltage is impressed. upon thegrid minimizing the surge andits effects."

The tubes inthe amplifier may be considered to act as a lowimpedancealternating current path, while atthe same time offering a highresistance to direct current.

The invention will, however, be better understood from the followingdescription when con sidered in connection with the accompanying drawingand its scope will be pointed out in the appended claims.

In the drawing, -Fig. 1 is a schematic circuit diagramof an amplifierembodying the invention and Fig. 2 is a further schematic circuitdiagram of an output amplifier for a radio receiver showing amodification of the invention.

1 Referring toFig. 1, an amplifier tube 5 is provided with an inputcircuit 6 for receiving audio frequency signals, and an anode outputcircuit 1 connected through an anode output resistor 8, andacouplingcapacitor 9, with an audio frequency output circuit I 0. i

1 Anodecurrent for the amplifier 5, is provided between a positivesupply lead H and a negative ground lead |2.

series filter resistor I3 is connected in circuit and is provided with aby-pass filter circuit I4,

to ground.

The filter circuit comprises a second amplifier tube l5 having a controlgrid l6, an anode l1, and a cathode l8. The latter is connected to theground lead l2, through a self bias resistor l9, and grid potential isapplied therefrom to the control grid l6, through a grid resistor 2|]connected between the grid and ground.

Potential surges in the supply circuit, causing a variation in thepotential between leads I l and i2, and surges caused by strong signalsapplied to the grid circuit 6, tending to make the anode current varyrapidly at audio and sub-audio frequencies are applied to the controlgrid of the regulating tube through a coupling capacitor 2|, connectedbetween the junction of the output impedance or resistor 8 and thefilter resistor l3 at the terminal 22.

The instantaneous surges are conveyed through the coupling capacitor 2|and cause the plate current of the regulating tube I5 to vary, therebytending to maintain the potential at the point 22 constant and opposingthe change which is occurring at that instant.

The reactance of the coupling capacitor 2| between the control grid N5of the regulating tube and the anode I! should be relatively low, whilethe grid impedance to ground provided by the resistor should berelatively high.

The variation in potential which may occur at the terminal 22 may berepresented by an alternating voltage E which is applied to the plate l1and to the grid l6 through the capacitor 2|. In the plate circuit of thetube the amplified grid voltage E, is effectively in series with theapplied voltage, the alternating component of the plate current will,therefore, be

and the impedance, in effect, will be In the positive supply leadbetween the output resistor 8 and said lead, a

The tube circuit l4 therefore takes the place of a by-pass capacitor andoperates to amplify the effect of the capacitor 2|- At a flutter freingupon the bias on the grid. With the same tube connected as shown in Fig.1, the plate circuit impedance to alternating current will be onlyone-fiftieth as much as the resistance to direct current. Because of thelow impedance between point 22 and ground, variations in the voltage ofthe anode of tube will cause the voltage to appear chiefly acrossresistor 8, and only a small voltage will occur between point 22 andground. Thus variations inplate current of tube 5 will cause onlysmallvariations in current through resistor i3, and the potential of thesupply voltage lead I! will be little affected. Likewise, any variationsin the voltage supply potential, caused by other amplifier tubes, wouldmade to drop most of the voltage in resistor 53, only small part of suchvariation in supply voltage would reach the anode i and succeeding partsof the amplifier through condenser Q and lead it.

Referring now to Fig. 2, a pair of balanced or push-pull connected audiofrequency amplifier tubes 25 and 25 are connected between a push-pull orbalanced input transformer ii? and a push-pull or balanced outputtransformer 28, to supply audio frequency power to a loudspeaker 29 whenaudio frequency signals are applied to the input winding 39 of the inputtransformer 2'5. The cathode heating supply circuit for the amplifier,indicated at 3!, is in common with a radio receiver 32, which may supplysignals to the input terminals 30, and is connected to ground, asindicated at 33, within a power supply device indicated at 34, whichreceives power from alternating current supply leads 35-35.

The power supply. unit 3 includes a rectifier indicated at 3'5 toprovide rectified and filtered direct current potential at suitableanode potential between output leads 3'! and 38, the final filteringbeing provided by a shunt by-pass filter capacitor 39.

The negative potential supply lead 38 from the rectifier is connected toground or chassis 39 through the loudspeaker field indicated at ill,which provides a filter choke coil in the negative lead and a source ofgrid bias potential for the tubes 25 and 26. A connection lead betweenthe negative terminal of the potential source 40 and a center tap $2 onthe input transformer is provided with a series filter resistor 43 forthe biasing potential for the tubes 25 and 25. A plate potential supplylead it is connected between a center tap 55 on the primary of theoutput transformer and the positive supply anode lead 31. A filtercapacitor 8% is also provided between the terminal 45 and the ground.

The radio receiver is also connected in common with the amplifier, tothe anode potential supply leads 3'? and 38, as indicated at M and asuitable filter capacitor is provided therefor as indicated at 48.

Variations in. anode potential caused by heavy load placed upon theoutput amplifier and audio orsub audio potential variations resultingtherefrom may be transmitted to the radio receiver and may adverselyaffect the operation of the tube circuits therein, as is well known. Toprevent this effect, the grid filter, including the series resistor 43,is completed by a filter capacitor 49 connected between the gridterminal 42 on the transformer 21 and the positive supply lead 3'?through a lead indicated at 50. With this connection, a signal surgetending to cause an increase in anode current through the tubes 25 and26, and a corresponding reduction in voltage on the positive lead 31, isconveyed through the filter or controlling capacitor 49 directly to thegrids of the amplifier tubes, causing said grids to become more negativeand this, in turn, tends to counteract the tendency of the anode currentto increase, destroys the effect of the surge on the supply circuit, andprevents the same from being transmitted, to the radio receivercircuits.

It will be appreciated that if the usual grid filter is provided for thepurpose of reducing the effect of ripple resulting from alternatingcurrent rectification, the time constant of the circuit so formed isrelatively slow and tends to maintain the grid bias potential constantduring the surge, thereby permitting the anode current to rise.

However, with the connections shown, the amplifier tubes are utilized toabsorb the alternating current or surge voltage. The circuit feeds backonly the undesired effects of the signal or surge and not the surgeitself, since the degenerative feedback connection is provided through areactance device of low impedance to alternating current, whilepreventing the flow of direct current because of the high direct currentresistance.

In an amplifier as shown, using RCA 2A3 tubes and a potential of 300volts between the leads 3? and 38, a surge conducting capacitor 49 of.25 microfarad is provided in connection with a filter resistor 43 ofsubstantially 56,000 ohms. As in the preceding example, the capacitor 49should have a relatively low reactance, while the grid filter resistor43 should have a relatively high resistance. For example, in the circuitof Fig. 1 the capacitor 2| may have a value of 1.0 microfarad and theresistor 2|) may have a resistance of substantially 1 megohm, therequirements depending upon circuit and the tubes used.

It has been found preferable, in commercial radio receivers for highfidelity output, to separate the balanced grid circuit from the filternetwork by introducing a resistor 43a between the mid-terminal of theinput transformer and the mid-tap on the filter, between the resistorand the capacitor. This may have a resistance of 100,000 ohms with 2A3output tubes as shown and is preferably higher in resistance than theresistor 43.

I claim as my invention:

1. In an audio frequency amplifier, a filter circuit comprising a filterresistor and an electric discharge amplifier tube, means providing arelatively high impedance path between the input electrode thereof andthe cathode, and a capacitor providing a relatively low reactance pathbetween the anode and said input electrode and providing the soleimpedance element in circuit between said electrodes, said anode beingconnec'tedto a point in said amplifier at which surges tend to develop.

2. In an audio frequency amplifier of the pushpull type having abalanced input circuit provided with a connection for grid biaspotential supply, means providing a balanced output anode circuit iorsaid amplifier having a centertap connection, means for applying audiofrequency and sub-audio frequency voltages to the 5 control electrodesof the push-pull amplifier simultaneously in parallel, said meanscomprising a capacitor of low reactance at said frequencies connected asthe sole impedance element between the anode circuit center-tap con- 10nection and said bias potential supply connection, and means forsupplying a biasing potential to said first-named connection comprisinga filter resistor having a relatively high resistance with respect tothe reactance of said capacitor.

5 3. In an audio frequency amplifier of the balanced type including apair of electric discharge amplifier tubes in push-pull relation to eachother, the combination of anode potential supply means for said tubescomprising a positive supply lead 20 and a negative supply lead, meansproviding a balanced input circuit for said amplifier having a biaspotential supply connection, means providing a balanced output anodecircuit for said amplifier having a center-tap connection with 25 saidpositive supply lead, a capacitor having a relatively low reactance toaudio frequency currents and sub-audio frequency currents connected asthe only impedance element in circuit between the said center-tapconnection and said bias potential supply connection, a filter resistorconnected between said capacitor and a point of negative biasingpotential supply in said negative supply lead, and means for isolatingsaid center-tap connection from said last-named point of connectionbetween said resistor and capacitor comprising a seriesresistor having aresistance at least as great as said first-named resistor.

4. In an audio frequency amplifier of the balanced type, comprising apair of electric discharge amplifier devices connected in balancedrelation to each other, the combination of an input transformer for saidamplifier having a secondary winding provided with a bias potentialsupply connection, an output transformer having a primary windingprovided with a centertap, a capacitor providing the only impedance incircuit between said bias potential supply connection and saidcenter-tap and providing a path of low reactance for audio frequencycurrents therebetween, means providing a source of biasing potential forsaid amplifier devices, and a resistor connected between the center-tapof the input transformer secondary and said source, said resistor havinga relatively high resistance with respect to the reactance of saidcapacitor.

WINFIELD R. KOCH.

